שָׁאַג

From LSJ

Ἅγιος ὁ Θεός, Ἅγιος ἰσχυρός, Ἅγιος ἀθάνατος, ἐλέησον ἡμᾶς → holy God, holy Mighty, holy Immortal, have mercy on us

Source

English (Strong)

a primitive root; to rumble or moan: X mightily, roar.